1. Field of the Invention
The present invention relates to modulators. More specifically, the invention relates to various improvements in modulators such as that disclosed in U.S. Pat. No. 4,804,931. As such, the present invention is especially useful in amplitude modulators that are capable of generating pseudo-continuous amplitude modulation at any modulation index between 0 and 1 with any vast array of carrier frequencies using almost any class of amplifier.
2. Related Art
In U.S. Pat. No. 4,804,931 (which is incorporated herein by reference in its entirety), a digital amplitude modulator-transmitter is described. An amplitude modulator-transmitter embodying the teachings of the '931 patent is illustrated in FIG. 1.
The amplitude modulator 30 includes a predetermined number of quadrature hybrid power devices 32.sub.l -32.sub.n. The quadrature hybrid power devices 32.sub.l -32.sub.n are configured as combiners, and are arranged in cascade so that the output of one combiner is the first of two inputs of a successive combiner. In the exemplary illustration of FIG. 1, only four combiners are shown for ease of understanding.
The construction and operation of a quadrature hybrid device is described in detail in the '931 patent, and need not be described here. However, for purposes of the present discussion, a quadrature hybrid combiner may be implemented as a four-port device having first and second inputs, one output, and one isolated port. The first and second inputs receive two signals of a given amplitude that are 90.degree. apart in phase. The input signals are combined to provide an output signal having an amplitude that is the sum of the amplitudes of the input signals. As shown in FIG. 1, ports 1 and 4 represent the first and second input ports. Port 3 represents an isolation port to which is attached a resistor representing a dummy load 34. Finally, port 2 defines the output port of each of the combiners.
An analog signal source 36, such as a microphone or a video generator, produces an analog signal that passes through an analog-to-digital converter (A/D converter, or ADC) 38. The digital output of the A/D converter appears on data lines 40.
As an example, there are four data lines, so that data consists of four-bit words. However, it is contemplated that 12- or 16-bit words may be employed to improve the quality of the signals being transmitted. Further, it is contemplated that a digital input signal may be fed directly to gates 42 without the need for A/D converter 38, should the modulating signal already exist in binary form.
Each of the bits, from the least significant bit (LSB) to the most significant bit (MSB), controls a respective gate 42.sub.l -42.sub.n. Each of the gates is connected in series with a corresponding amplifier 44.sub.l -44.sub.n. The series combinations receive an RF signal from an RF signal generator 46, and provide respective second inputs to the combiners.
The output port of the LSB combiner 32.sub.1 represents the least significant bit of the digital word describing the instantaneous value of the signal to be modulated. The second input of the combiner 32.sub.2, adjacent the LSB combiner 32.sub.1, represents a value that is twice as significant as that of the second input of the LSB combiner 32.sub.1. Similarly, the second input of each successive combiner represents a value twice as significant as that input to the immediately preceding combiner.
Thus, the output of each combiner represents a binary weighted power signal. In particular, the signal appearing at the output of the MSB combiner 42.sub.n represents the sum of the power signals fed into the various combiners. This output signal is fed for transmission to a load, typically an antenna with conventional filtering.
The power efficiency of the digital amplitude modulator-transmitter is described in equation 24 of the '931 patent: ##EQU1## where m.sub.k is the modulation index (0=off, 1=on) of the k-th bit of the binary signal on path 40 and n is the number of bits in the word. Specifically, this is the power efficiency of the combining network with intentionally wasted power in the combiner reject loads when all RF driving sources are not gated ON (that is, when all m.sub.k are not unity). It is this intentional waste of power that causes the system in the '931 patent to be a linear amplitude modulator-transmitter.
The Applicant of the present patent application has realized that wasted power should be minimal at the highest capable peak power of the modulator-transmitter. In fact, when all driving sources (gates 42) ON (all m.sub.k =1), power efficiency is 100% in the combiner. However, combiner power efficiency cannot readily be improved if the modulation source is completely random, unpredictable and aperiodic. This is seldom the case, however, and it will be shown below with reference to a first aspect of the present invention, that significant power efficiency increases may be achieved when occurrences of peak power are predictable and frequent.
On a second matter relating to amplifiers in the '931 patent and amplifiers in general, it is desirable that output power be maintained constant, at least for a given time period. However, the magnitude of output power of RF amplifiers may drift for a variety of reasons, including variations in the temperature of the device, changes in power supply voltage, and load impedance. Despite these confounding influences, it is desirable from a modulation standpoint to maintain the output power of the amplifiers constant during periods in which they are turned on. A second aspect of the present invention is directed to an improvement which fulfills this need.
Third, it is desirable to minimize component count and circuit complexity, while serving as many of the above-mentioned needs as possible. A third aspect of the present invention fulfills this need.